HMI Flicker With The Alexa & Epic

I've been shooting a lot of high speed content with the Alexa and the Epic lately and I have uncovered some unexpected results when shooting with both cameras. Watch the 2 minute video, and then read on to learn more about what is going on.When I completed my testing of the Epic with magnetic ballast HMI's my results were as I expected - multiples of 60 were the most flicker free settings to use, and in general it is best to avoid magnetic ballast HMI's. So it surprised me when I saw filker form the Epic when I was shooting at supposedly safe settings using flicker free electronic ballast HMI's. People like Art Adams have explained and documented that a frame rate of 23.98 and a relative shutter speed of 180 degrees is not flicker free. Applying his experience, and making choices based off of well published charts is a safe place to start from, but the final word on the matter can only come through hands on testing and experimentation as I found out on this production. Regardless of what the numbers say, always test and verify.

What I find perplexing:- According to the charts, if I am shooting at 120 fps, I should be able to use any shutter speed I want. However, with the Epic, in the example above, I was only able to get a flicker free image at 360 degrees*.

- If the flicker in the Epic was a result of the 23.98 project time base, then I should still see the flicker in the Alexa as it had the same time base at 120 fps.

*This was a commercial shoot where I only had 4 hours with the talent, so it was not an ideal situation to test and compare. As such, I did not record a bunch of samples to evaluate later. After we discovered this issue, we dialed in the shutter until we found a flicker free setting. The Epic was running firmware 2.0.5.

Possible Explanations:- There was an issue with the bulbs or the ballasts. I cannot entirely rule this out, as I did not have every unit inspected when we saw the issue due to our time constraints on set. I have my doubts about this being the issue, as the rental house I work with does a great job at maintaining their gear. Furthermore, if this was the issue, then I would expect that it would also appear on the Alexa footage.

- There was an issue with the mains power. This cannot be completely ruled out either, as the location of the shoot with the Epic and the location of the shoot with the Alexa were different. However, the Epic shoot was in a newer building, and the Alexa shoot was in an older gutted building, so if this was the issue, I would expect it to be in the Alexa footage, not the Epic footage. Furthermore, the whole point of having a ballast, and one that does flicker free, is to control the power to prevent these kind of issues from appearing.

- The way the sensor is made and processed by the cameras. In my opinion, this seems to be the most likely answer. While both cameras have a single CMOS sensors, each chip is custom made and the internal processing is completely different. I think this is the most likely variable, as I have encountered HMI flicker on the Red One before.

Recommendations:- Always test and verify. Don't take my word for it. Maybe what I encountered was a one time event that happened when all of the stars aligned perfectly to cause this issue. (I have my doubts about that ...) But you'll never know for sure if the numbers are correct until the actual recipe of camera system + camera settings + lighting package is made.

- Use a tungsten lighting package with the Epic / Scarlet / Red One. If at all possible, stay away from HMI lights unless you've done the proper testing. This is especially true if you are shooting at off speeds. As you approach high frame rates, you'll need to use 5k bulbs or bigger, as you will begin to see the flicker of the AC cycle in smaller bulbs. The larger bulbs do not have enough time to cool down in the cycle to reveal themselves. (If the light can be powered via DC, then smaller bulbs should not be an issue.)

- If you are shooting with the Epic or the Scarlet and HMI lights, always use electronic flicker free ballasts and set the camera to an absolute shutter. If you are seeing flicker in the image, dial in a shutter speed that is a multiple of 60. If that isn't working, try 360 degrees, as that seemed to work for me.

I look forward to being able to explore this issue in a more controlled environment in the future. Hopefully then I can have a more definitive answer to what is going on between these cameras.

Until Next Time - Get Out There And Shoot!Ryan E. Walters, Cinematographer